1. Field of the Invention
The present invention relates to semiconductor devices including power devices, and more particularly, to a semiconductor device including an inverter switching element.
2. Description of the Related Art
FIG. 7 is a cross sectional view of a power semiconductor device described in JP, 10-512450, A and indicated in its entirety as 500. The semiconductor device 500 includes a metal substrate 501. A circuit portion is disposed on the metal substrate 501.
The circuit portion includes an insulation substrate 502. A semiconductor element 504 is fixed on the insulation substrate 502 by means of a solder layer 503. Leads 505 are connected to an electrode of the semiconductor element 504 by bonding wires 506. The semiconductor element 504 and the like are sealed up in mold resin 507. The circuit portion is fixed on the metal substrate (base plate) 501 by means of a solder layer 508.
Further, a control circuit board 510 is disposed on the metal substrate 501, while a conductor 511, a relay terminal 512 and the like are disposed on the control circuit board 510.
In addition, a relay substrate 520 is disposed on the metal substrate 501, and a conductor 521 is disposed on the relay substrate 520.
The leads 505 of the circuit portion are connected with a main circuit terminal 530 and a control terminal 531 through the conductor 511.
Such a semiconductor device 500 is used with a heat sink (not shown) attached to a back face of the metal substrate 501. Typically grease (not shown) is thinly applied between the metal substrate 501 and the heat sink. As screws are turned into screw holes formed in the metal substrate 501, the heat sink and the metal substrate 501 get fixed (not shown).
However, since the circuit portion and the like are disposed on the expensive metal substrate 501, it is difficult to cut costs of manufacturing the semiconductor device 500. When a large current is to be carried by a circuitry wire disposed on the metal substrate 501, since the circuitry wire needs to be large not only in the thickness direction but also in the width direction, the size of the metal substrate 501 can not be reduced.
Meanwhile, efficiency of heat dissipation from the semiconductor device 500 is largely dependent upon the level of adhesion between the metal substrate 501 and the heat sink. The metal substrate 501 is designed in advance in such a manner that the metal substrate 501 has a flat back ace as a finished product mounting the circuit portion 550 and the like. The back face of the metal substrate 501 is not to be processed flat after completion of the final product.
Hence, the surface area of the metal substrate 501 is large at the time of assembling more than one circuit portion into the semiconductor device 500, and therefore, it is difficult to ensure the flatness of the back face.
On the other hand, when the semiconductor element 504 is fixed directly on the metal substrate 501, the mold resin 507 warps. That in turn leads to a variation in film thickness of the solder layer connecting the semiconductor element 504 and the mold resin 507 to each other, causing a drop in reliability and a variation in thermal resistance.
In addition, there is a problem that the tight adhesion between the metal substrate 501 and the mold resin 507 becomes insufficient and dielectric breakdown accordingly occurs.
Further, when the locations of the main circuit terminal 530 and the like are to be changed, it is necessary to change the whole design of the semiconductor device 500 including the circuit portion.